Coated electroluminescent (EL) devices are a promising technology for flat-panel displays and area illumination lamps. Advances in EL devices, particularly within the subcategory of organic light-emitting diodes (OLEDs), are making this technology competitive with traditional LCD technology for display and tungsten or fluorescent bulbs for area illumination. This technology relies upon thin-film layers of materials coated upon a substrate. In OLED devices, these materials are organic but EL devices may also be formed from inorganic or combinations of organic and inorganic layers.
OLED devices generally can have two formats known as small-molecule devices such as are disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as are disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. In most designs, one of the electrodes is reflective and the other transparent. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron-transport layer (ETL) and the hole-transport layer (HTL) and recombine in the light-emissive layer (LEL). Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of LEL can determine how efficiently the electrons and holes can be recombined and result in the emission of light, etc.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer and transparent cover. Light generated from the device is emitted through the top transparent electrode and transparent cover. This is commonly referred to as a top-emitting device.
OLED devices can employ a variety of light-emitting organic materials patterned over a substrate that emit light of a variety of different wavelengths, for example red, green, and blue, to create a full-color display. Alternatively, it is known to employ a combination of emitters, or an unpatterned broad-band emitter, to emit white light, together with patterned color filters, for example red, green, and blue, to create differently colored light emitting elements and a full-color display. The color filters may be located on the substrate, for a bottom-emitter, or on the cover, for a top-emitter. For example, U.S. Pat. No. 6,392,340 entitled “Color Display Apparatus having Electroluminescence Elements” issued May 21, 2002 illustrates such a device.
OLED materials have different light-emission properties, and it is known that some colors of light are emitted more efficiently than others, in particular white emitters are known that have an efficiency higher than that of both blue and red emitters. Hence, it has been proposed to employ OLED pixels having four sub-elements: red, green, blue, and white (RGBW). Because most images have a large luminance component, such four-element displays can be more efficient than conventional three-element displays. Such designs are described, for example, in U.S. Pat. No. 6,919,681 and US2004/0113875. In some cases, the white emitters may be more efficient than the color emitters, hence power usage may be reduced by using the white emitter to replace a portion of the light emitted by a combination of the colored emitters. Additionally, it is known that the application of a fourth gamut defining color light-emitting element having a power efficiency higher than at least one of the RGB light-emitting elements can also improve display efficiency as described in US20040178974.
Referring to FIG. 3, a side view of a bottom-emitting OLED device as suggested by the prior art is illustrated having a transparent substrate 50. Over the substrate 50, a semi-conducting layer is formed providing thin-film electronic components 24 for driving an OLED. An interlayer insulating and planarizing layer 40 is formed over the thin-film electronic components 24 and a patterned transparent electrode 52 defining OLED light-emissive elements is formed between the insulating layers 40. An inter-light-emitting element insulating film 42 separates the elements of the patterned transparent electrode 52. One or more first layers 54 of organic material, one of which emits light, are formed over the patterned transparent electrode 52. A reflective second electrode 56 is formed over the one or more first layers 54 of organic material. In some prior-art embodiments, the patterned transparent electrode 52 may instead be at least partially transparent and/or light absorbing.
As shown in the RGBW configuration of FIG. 3, the organic layers 54 need not be patterned, and broadband light, for example white light, can be emitted from the organic layers 54, through the color filters 44R, 44G, and 44B to form a color display. The color filters 44R, 44G, and 44B, transmit only a desired color of light, for example red, green, and blue corresponding, for example to the desired colors in a color display while filter 44K is a neutral density filter (or a transparent layer, no filter, may be present). The colored light 60, 61, 62 and broadband, or white, light 63 then passes through the substrate 50 and is emitted from the device. Alternatively, the organic layers 54 may be patterned so that differently colored light is emitted from different locations on the device and no filters 44R, 44G, and 44B need be employed. The Applicant has constructed an OLED device, corresponding to FIG. 3.
Unlike other flat-panel display devices, active-matrix OLED devices employing thin-film electronic components to drive the OLED elements generally suffer from problems with power distribution in the OLED device. Since the OLED devices are directly emissive and the light output is dependent on the current passing through the OLED, it is important that adequate and uniform current be available to each OLED in an active-matrix OLED device.
OLED devices are traditionally laid out with rows and columns of pixels and either column or row buses supplying power to each OLED pixel so that each column (or row) of pixels share a common bus (as well as other signals such as data and select). This arrangement is employed to simplify the layout of the pixels and maximize the resolution of the OLED device. In an active-matrix OLED, a thin-film transistor circuit is provided that regulates the current provided to each OLED within the display device. However, current is typically commonly provided to a large number of pixels in a row or column by a power and return line pair. Because these lines have a finite resistance, as the current that is required to drive each OLED element increases, an unintended and undesirable voltage differential is produced as a function of the current that is drawn and the resistance of the power and return line. Since the unintended voltage differential is positively correlated with current and resistance, the loss of voltage along the power and/or return line will be large when the lines must deliver high currents or when the power line has a high resistance. The phenomena that produces this unintended voltage differential is commonly referred to as current-resistance or “IR” drop. Further, this IR drop will result in the gradual loss of luminance for pixels along a power line as the distance from the power source increases. This loss of luminance has the potential to create undesirable imaging artifacts. Therefore, there is a need to limit unintended voltage drop to avoid these artifacts. Referring to FIGS. 6A and 6B, diagrams are presented illustrating an actual image artifact observed in a commercially-available active-matrix OLED. Area 1 is brightly illuminated and therefore draws a significant amount of power. Area 2, including portions 2a, is less brightly illuminated with a common signal and is located farther from the power source than area 1 so that current must pass a greater distance through the column buses found in the active-matrix OLED employed for the demonstration. Although area 2a is driven with the same signal used to drive the remainder of area 2, the current in area 2a is differentially limited by the current drawn along the column bus in area 1 and therefore exhibits a lower luminance than in the remainder of area 2, leading to an observable, unintended, and undesirable image artifact. In particular, the artifact is column-structured and is defined by the layout of the power and return line within the display device.
One method to overcome this problem is to reduce the resistance of the power lines as suggested in US 2004/0004444, entitled “Light-emitting panel and light-emitting apparatus having the same”. Buses are typically constructed, for example, of aluminum or silver or other metals or metal alloys and may be, for example, 10 microns wide and 250 nm thick. Unfortunately, since the materials that are available to reduce resistance and the cross-sectional area of the power line, which is inversely proportional to resistance, is fixed by the manufacturing technology that is available, it is often not cost effective to reduce the resistance of the power line. Since resistance increases with increases in the length of the power line and the peak luminance of the display is limited by the peak current that can be provided along a power line, this phenomena often limits the size or luminance of displays that can be produced using OLED technology.
Another prior-art method to address this problem relies upon increasing the size of the power-distribution buses at the expense of other display elements. For example, the buses can be increased in size (and their conductivity improved) by reducing the size of the emissive areas of an OLED (the aperture ratio). However, this reduces the lifetime of the OLED device since the current density through the OLED materials is increased (at a given luminance) and it is known that the lifetime of an OLED is inversely related to the current density passed through the OLED. Referring to FIG. 4A, a top view of a conventional prior-art OLED layout is shown comprising two neighboring pixels 10a and 10b, with light-emissive areas 20a and 20b, thin-film electronic (TFT) electronic components 24a and 24b and buses 14a and 14b. Because of manufacturing process limitations, the precision with which the various components can be located is limited; therefore spacing 8 must be employed between the various device elements. A great variety of layout design rules and layers is known in the art and define the spacing 8 that may be considered for the various device elements. The figures shown are a very simplified and abstracted illustration. In particular, note the spacing 8 adjoining the buses 14; two spaces 8 are employed within each pixel 10. U.S. Pat. No. 6,522,079 describes an OLED layout with improved bus width without reducing the aperture ratio. Referring to FIG. 4B, by reducing the number of buses, the number of spaces 8 can be reduced to three between every two pixels. The additional space 14′ can be employed to increase the size of the remaining bus (as shown) to improve conductivity and power distribution, or for other purposes such as increased aperture ratio and OLED device lifetime. U.S. Pat. No. 6,771,028 describes drive circuitry for four-color organic light emitting devices, and similarly includes reflected layout embodiments where adjacent columns of light-emitting elements share a common electrical bus. Despite this increase in size and conductivity enabled by such layouts, power distribution remains a significant problem and further improvements would be desirable.
Referring to FIG. 4C, another approach to improving the power distribution of an OLED device employing row or column buses is described in U.S. Pat. No. 6,724,149. This design uses bypass cross-connectors 12a and 12b electrically connecting buses 14 to allow current from one bus 14 to reach another. However, use of such cross connections between each row of light-emitting elements may significantly reduce the aperture ratio of an OLED device, particularly in a bottom-emitter OLED configuration.
Because all EL devices, including OLED devices, produce light as a function of the current that passes through the device, all active matrix EL devices exhibit similar problems. There is a need therefore for an improved EL device layout that avoids the problems noted above, providing either a larger area for power bus lines and/or emitting area while reducing the visibility of artifacts.